The primary function of a defibrillator is to provide a high-energy electrical pulse to a patient's heart. As such, a defibrillator will typically have a high-voltage charger, a storage capacitor, and a pair of defibrillation contacts. A plurality of switching elements, commonly being mechanical relays, alternately connect the storage capacitor to the high-voltage charger and the defibrillation contacts. During discharge, the patient must be isolated from the high-voltage charger circuit. To prevent the storage capacitor from accumulating a charge, a safety resistor can be placed in parallel with the storage capacitor.
A defibrillator may have a "pacing" circuit that periodically provides small current pulses to the patient's heart. The pacing circuit uses the same defibrillation contacts as are used for defibrillation. However, during discharge the pacing circuitry must be disconnected from the defibrillation contacts or it can be destroyed.
A first prior art patient circuit configuration is shown in FIG. 1. A pulsed-DC power supply 10 provides energy to a storage capacitor 13 through a step-up transformer 11. A diode 12 prevents the charge accumulated on the storage capacitor from dissipating. To discharge the accumulated energy to the patient, modeled in the drawing as a resistor 18, the double-pole relay 14a, 14b is switched. An inductor 15 regulates the current flow to the defibrillation contacts 16 and 17 and the patient 18.
Although functional, the patient circuit shown in FIG. 1 has some potential problems. First, if the series RLC circuit comprising the storage capacitor 13, the inductor 15, and the patient 18 is underdamped (a condition which is dependent on the patient's transthoracic impedance), then the voltage across the capacitor can go negative, forward biasing the diode 12. This "undershoot" condition" can impose a high voltage across the secondary of the step-up transformer 11 with only a small series resistance in the circuit, destroying the transformer secondary.
Second, if a pacer circuit (not shown) is connected to the defibrillation contacts 16 and 17, an additional relay is needed to protect the pacer circuit during discharge.
Third, if the relays 14a and 14b are switched from their discharge positions while current is still flowing through the inductor 15, the relays can "arc over." Because of the physical arrangement of the relays 14a and 14b in a double-pole double-throw configuration, the arc-over effectively shorts the terminals of the storage capacitor 13, destroying it.
A second prior art patient circuit configuration is shown in FIG. 2. As in FIG. 1, pulsed-DC power supply 20 provides energy to a storage capacitor 23 through a step-up transformer 21. A diode 22 prevents the charge accumulated on the storage capacitor from dissipating. To discharge the accumulated energy to the patient 28, the double-pole relay 24a, 24b is switched. An inductor 25 regulates the current flow to the defibrillation contacts 26 and 27 and the patient (modeled by a resistor 28).
This configuration for a defibrillator patient circuit shares many of the potential problems as that shown in FIG. 1. For example, it is susceptible to an arc-over condition destroying the storage capacitor 23. Also, an additional relay would be required to isolate a pacer circuit (not shown) during discharge.
The arrangement of the relays in FIG. 2 does protect the secondary of the transformer 21 from an undershoot condition.
A third prior art patient circuit configuration for a defibrillator is shown in FIG. 3. As in the prior two figures, a pulsed-DC power supply 30 provides energy to a storage capacitor 33 through a step-up transformer 31. A diode 32 prevents the charge accumulated on the storage capacitor from dissipating. To discharge the accumulated energy to the patient, modeled in the drawing as a resistor 38, the double-pole relay 34a, 34b is switched. An inductor 35 regulates the current flow to the defibrillation contacts 36 and 37 and the patient (modeled by a resistor 38).
This configuration shares the problems experienced with the patient circuit configuration shown in FIG. 2. However, the inductor's position between the relays 34a and 34b protects the storage capacitor 33 in an "arc-over" condition.
For the foregoing reasons, there is a need for a patient circuit configuration which (1) isolates the patient from the high-voltage charger circuit; (2) isolates a pacer from the storage capacitor during discharge; (3) isolates the high-voltage charger circuit from the storage capacitor during discharge; (4) minimizes the risk of relay failure due to interruption of discharge; while (5) minimizing the number of relays used.